Upon cophasal energization of the radiators of the several quadrants of such an antenna during a transmission phase, echos reflected by a target offset from the axis of the array will give rise to phase differences in the outputs of the antenna quadrants during a reception phase from which the location of the target (in terms of azimuth and elevation) can be determined by addition and subtraction. By varying the operating frequency of the transmit/receive unit, and therefore the wavelength of the radiated energy, these parameters can be determined with great exactitude.
It is known to design the radiators of such antennas as waveguides provided with longitudinal slots whose center-to-center spacing or pitch equals half the natural wavelength .lambda..sub.go of the guide. For operating wavelengths deviating significantly from .lambda..sub.go, the waves radiated from different slots are no longer in phase (or phase opposition) so that a substantially planar wavefront cannot be maintained. This problem is aggravated as the length of the radiators is increased to enhance the power of the antenna.